Steady-state octanoate blocking affinities (at −140 mV) of various pore-open mutants of CLC-0. The octanoate affinities of various mutants are approximately threefold higher than the CPA affinities in Fig. 3 B, except for the four mutants labeled by red solid squares. Also notice that in this plot, the hydrophobic (with a positive hydrophobic index) and the hydrophilic (with a negative hydrophobic index) residues are easily separated into two distinct groups.